1. Field of the Invention
The present invention relates to a semiconductor wafer that includes a dot mark having a peculiar shape and formed on a part of the wafer. More particularly, it relates to a semiconductor wafer including a dot mark having a shape that is excellent in optical visibility and enables the azimuth of a crystal axis of the semiconductor wafer to be optically recognized. It also relates to a method of forming such a dot mark.
2. Description of the Related Art
Electric characteristic of silicon, which is a substrate material of a semiconductor integrated circuit, depends on the azimuth of a crystal axis thereof. Therefore, when a circuit is printed onto a silicon wafer, which is a substrate material of a general semiconductor, its circuit pattern has to be matched with the azimuth of a crystal axis of the material. Thus, conventionally, a mark indicating the azimuth of the crystal axis is made on the semiconductor wafer.
As a typical example of this mark, an orientation flat is formed by cutting out part of a disc-shaped semiconductor wafer along a chord of the semiconductor wafer, which is perpendicular to the azimuth of its crystal axis. Generally, such an orientation flat is applied to a semiconductor wafer 150 mm in diameter. Sometimes it is also applied to a wafer 200 mm in diameter. Recently, as the semiconductor wafer to be used has been enlarged (diameter: 200 mm or more), a V-shaped notch has been applied for a mark. The V-shaped notch is formed in part of a periphery of a semiconductor wafer such that the azimuth of its crystal axis is matched with a line connecting a vertex of the V-shaped notch with a center of the wafer. This is because a demand of the manufacturers who want to obtain as many semiconductors integrated circuits as possible by enlarging the semiconductor wafer. Furthermore, minute disparity in film-forming quality during formation of the circuits due to the orientation flat has significantly influenced upon the degree of integration.
However, the azimuth mark of the aforementioned notch can also make a disadvantageous influence upon the degree of integration. Furthermore, because the notch forms a minute space, dust such as contamination or the like tends to be accumulated in the notch. Therefore, considering such influence, there has been a trend that the azimuth of the crystal axis on the semiconductor wafer is indicated by marking with a laser marker, instead of the aforementioned markings. However, marking of the azimuth of the crystal axis by the laser marker has not been so standardized as the aforementioned marking technology because it accompanies change of the existing equipment and leads to an increase of production cost.
On the other hand, through a manufacturing process for a semiconductor wafer and a semiconductor device, a laser marker is often used for marking management information including ID information, processing history and electric characteristics on part of the semiconductor wafer surface. Therefore, if a mark formed by the laser marker only indicates the azimuth of the crystal axis of the semiconductor wafer directly, the azimuth of the crystal axis need not to be measured by preliminarily using an X ray. Further, the semiconductor wafer does not need to be cut out at all. Therefore, this method can satisfy both the demands of the wafer manufacturers and the semiconductor device manufacturers.
The present invention has been achieved in views of these circumstances. Specifically, an object of the present invention is to provide a semiconductor wafer having a single azimuth mark or plural azimuth marks, which is not affected by cutting out and enables the azimuth of a crystal axis to be recognized by the mark or marks, by means of combining improved laser marking technology with ordinary general processing technology. Furthermore, the present invention has an object to provide a method of forming a mark having such peculiar characteristics.
The inventors of the present invention proposed a dot mark having a peculiar shape, which is different from a conventional dot mark of a concave hole type dot mark formed by means of conventional laser marking technology, and a forming method thereof through Japanese Patent Application No. 10-334009. The dot mark according to the invention of this prior application is formed by marking a surface of a product to be marked by laser beam as an energy source. A center portion of each of the dot marks has a protruded portion which is protruded upward from the surface of the product to be marked. This is a very fine dot mark having a length of 1 to 15 xcexcm along the marking surface, and a height of the protruded portion in a range of 0.01 to 5 xcexcm. This dot mark has an optically excellent visibility in spite that it is such a very fine dot mark.
When the inventors formed a thin film on the mark forming surface of the semiconductor wafer which has the aforementioned dot mark with such a protruded portion by expitaxial growth, they found that its initial dot shape had changed to a different one. Then, the inventors repeated experiments by changing a length of the dot mark along the marking surface and changing a thickness of a single crystal by the aforementioned epitaxial growth. Consequently, it has been found that if the thickness of the grown crystal is within an appropriate range, the dot mark grows to a polygonal pyramid or truncated polygonal pyramid having clear ridge lines. Furthermore, when the inventors formed a single crystal by epitaxial growth after forming the plural dot marks on the surface of the semiconductor wafer, they found that the respective dot marks have the same shapes after their shapes are changed and further, the corresponding ridge lines are oriented in the same direction.
The inventors further considered that the ridge lines were somehow related to the azimuth of the crystal axis. Therefore, they measured the azimuth of the crystal axis in the semiconductor wafer after the epitaxial growth. Consequently, they found that the ridge lines coincide with the azimuth of the crystal axis completely. Although a cause of such a change of the dot mark shape is not proved, the epitaxial growth apparently grows a crystal having the same surface azimuth as that of a substrate surface on the single crystal substrate. Because characteristics such as atomic density differs depending on the surface azimuth of the substrate, the growth velocity has an anisotropy of the growth that differs depending on the surface azimuth.
Therefore, it can be considered that the velocity of the epitaxial growth at a minute point protruded from the surface of the substrate differs depending on the surface azimuth. Therefore, the minute point will grow to a polygonal pyramid having ridge line extending along the azimuth of the crystal axis. From this estimation, it can be considered that the dot mark formed on the semiconductor prior to the epitaxial growth does not always have to be formed by a laser marker but may be formed by chemical vapor deposition method or the like, which also makes it possible to form a dot mark having a protruded part from the dot mark forming surface.
The present invention has been based on the above described knowledge. According to a first aspect of the present invention, there is provided a semiconductor wafer including a dot mark protruded from a surface of a semiconductor wafer at an arbitrary position, wherein the dot mark has a shape that allows an azimuth of a crystal axis to be optically recognized.
Specifically, the dot mark having the aforementioned shape is formed at an arbitrary position of the semiconductor wafer so that the azimuth of the crystal axis can be recognized directly from the shape of the dot mark. Therefore, it is not necessary to form an orientation flat or a V-shaped notch especially after the azimuth of the crystal axis is measured. Thus, not only an apparatus for measuring the azimuth of the crystal axis becomes unnecessary, but also the cutting part can be eliminated, which makes it possible to efficiently obtain a number of integrated circuits.
Further, because the dot mark for indicating the azimuth of the crystal axis has no locally deformed portion due to the orientation flat and V-shaped notch, no dust is accumulated even through multiple steps processes, so that cleanness can be maintained.
Furthermore, according to the first aspect of the present invention, it is preferable that the dot mark includes a ridge line indicating the azimuth of the crystal axis. As described previously, the dot mark formed on the semiconductor wafer after the epitaxial growth has clear ridge lines, which are oriented along the azimuth of the crystal axis of the wafer. Therefore, the azimuth of the crystal axis can be recognized easily by specifying the azimuth of the ridge lines.
Still further, according to the first aspect of the present invention, the shape of the dot mark is specified. Namely, it is preferably a polygonal pyramid or a truncated polygonal pyramid each having polygonal faces. If the number of polygonal faces is even, a diagonal line passing a center of the mark is a straight line so that the azimuth of the crystal axis can be recognized easily. Further, the number of the polygonal faces is preferably six or less. Further, it is more preferable that it has four faces. In this case, the ridge lines are disposed in a cross shape, so that when plural dot marks are formed, the ridge lines are arranged on a straight line, thereby making it possible to recognize the azimuth of the crystal axis indicated by the straight lines. If the number of the polygonal faces is eight or more, the entire shape almost becomes a circle, which makes it difficult to specify the direction of the ridge lines and the azimuth of the crystal axis.
Still further, according to the first aspect of the present invention, it is preferable that the dot mark is formed on part of a peripheral face of the semiconductor wafer. Front and rear sides of the peripheral face of the semiconductor wafer are partially chamfered. These chamfered regions of the front and rear sides and a side face region between these chamfered regions are very minute. Further, these regions are least affected by an interference with a surrounding apparatus or by chemical polishing in a wafer production process or a semiconductor device production process. Particularly, the front and rear side chamfered regions are stable in configuration. Therefore, if the aforementioned dot mark can be formed in these regions, it is never eliminated even through many manufacturing processes, so that the azimuth of the crystal axis can be always recognized whenever necessary. In order to form a desired number of dot marks in the aforementioned regions, the dot marks themselves are required to be fine. In this respect, the dot mark forming technology of the aforementioned prior invention proposed by the inventors is very effective.
According to a second aspect of the present invention, there is provided a method of forming at least one dot mark having a peculiar shape, comprising steps of: forming a dot-like mark having an arbitrary shape protruded from a surface of a semiconductor wafer at a predetermined position of the semiconductor wafer; forming a thin film composed of a single crystal on an entire surface of the semiconductor wafer by epitaxial growth; and during the epitaxial growth, converting the dot-like mark into a polygonal pyramid shape or a truncated polygonal pyramid shape each having polygonal faces and including a ridge line indicating an azimuth of a crystal axis.
The dot-like mark, which is to be formed prior to the epitaxial growth, can be formed easily by means of the laser marker according to the prior invention proposed by the inventors. Alternatively, the dot-like mark having such a shape can be formed by other processing technology, such as chemical vapor growth method. Further, the aforementioned epitaxial growth method in the present invention can employ a conventionally well known technology and it does not have to be modified especially for the present invention.
Furthermore, according to the second aspect of the present invention, it is preferable that a maximum length of the dot-like mark parallel to the surface of the semiconductor wafer is 1-15 xcexcm. Since the dot-like mark is so fine, a required number of the dot-like marks can be formed on a peripheral face of the wafer as described above. Further, this dot mark has an optically excellent visibility even after the epitaxial growth. Therefore, this can be used for various kinds of management information.